The present invention relates to a mechanism of how to create a mark such as a letter, numeric character, or symbol, to be inscribed to a semiconductor device and a method of inscribing the mark. More particularly, the present invention relates to a method of applying a laser mark onto an exposed back surface of a silicon chip, which has conductive balls serving as external contact terminals attached on a front surface, and which is mounted on a wiring substrate in a flip-chip manner.
A semiconductor device is usually produced through a designing step, a mask-formation step, a wafer manufacturing step for forming a wafer from an ingot, a wafer processing step for forming an inner circuit such as a semiconductor element or an integrated circuit on the wafer, an assembly step and an inspection step.
A mark such as a letter, numeric character, or symbol indicating a product name or a pin number is conventionally inscribed on the margin on a surface of a semiconductor chip. The mark is integrally formed as a part of the margin of an inner circuit pattern of a chip while the chip maintains a wafer form before scribing. Thus, generally, a back surface of the chip has not been used for marking.
However, there is another type of chip having conductive balls (serving as external contact terminals) fitted to a front surface which has an inner circuit, such as an integrated circuit or a semiconductor element, formed thereon. In this case, the chip is mounted on a wiring pattern of a circuit board with the conductive balls interposed therebetween. The chip of this type has a following problem. Although an exposed wiring portion is protected, the chip is not sealed with a resin. As a natural consequence, the back surface of the semiconductor substrate is not protected. In such a circuit device having a chip mounted on a circuit board in such a way that a front surface of the chip faces the circuit board, it is impossible to identify a product name of a chip mounted on the circuit board during inspection even if a malfunction of the circuit device takes place. To overcome this, a mark such as a letter, numeric character, or symbol comes to be inscribed on a back surface of the chip, namely, a back surface of a wafer (Japanese Patent Application KOKAI Publication Nos. 4-106960, and 8-191038).
The mark such as a letter, numeric character, or symbol formed on the wafer or the chip is conventionally created by continuously arranging a plurality of dots formed by a laser.
The conventionally-employed laser mark is inscribed by a laser such as a YAG laser on a surface such as a back surface of a silicon wafer or its front surface having an element formed therein, so that the mark can be visually recognized. When the mark is inscribed on a silicon chip, the laser is applied in such a way that dots are not linked with each other in order to reduce damage.
FIGS. 1A to 1C are cross-sectional views of a silicon wafer sequentially showing conventional marking processes. A silicon wafer 1, which is sliced from a rod, is subjected to a lapping operation, and then, sent to a wafer processing step. In this operation, an inner circuit such as a semiconductor element or an integrated circuit, is formed. Accordingly, the silicon wafer 1 has a back surface 3 rendered uneven to a predetermined level by the lapping operation, and a front surface 2 having a plurality of chip formation regions in each of which a semiconductor element and an integrated circuit are formed. The mark such as a letter, numeric character, or symbol is inscribed on, for example, the back surface 3 of the wafer 1 by using a YAG laser. Alternatively, the mark may be inscribed on a back surface of the chip after the wafer 1 is diced into chips. The mark is formed of combination of a plurality of dots 5 (a depth of about 5 .mu.m), which are formed by applying a laser beam 4 (a beam diameter of about 120 .mu.m) onto the back surface 3 of the wafer.
However, even if care is taken to avoid linkage of dots in order to reduce damage, the application of a laser beam during the mark inscription produces cracks in the wafer having a thickness of about 500 .mu.m at most. As a result, a mechanical strength of the wafer is inevitably reduced. When the marking is performed in this method, it is not possible to create a fine-logo mark. Therefore, the fine-logo mark must be omitted in some cases. Although it is possible to create dots continuously to create fine marks, the mechanical strength of a wafer significantly reduces.